1. Technical Field
This invention relates generally to shaving devices, and more specifically to razor cartridges having a pivotal connection to handles of safety razors.
2. Background Information
In recognition that skin surfaces to be shaved are not planar, many modern shaving implements, also commonly known as wet shave or safety razors, include a disposable razor cartridge releasably connected to a reusable handle and adapted to be pivotally connected to the handle. The cartridge comprises a housing having at least one razor blade with a sharpened cutting edge disposed therein. Other modern safety razors have a handle and a pivotally connected razor cartridge that are intended to be permanently coupled and disposed of as a single unit. During use, the razor cartridge can pivot relative to the handle about a pivot axis between a neutral, or at-rest, position and a rotated position. The razor cartridge may be adapted to pivot in one direction only away from the neutral position or in two directions. The ability of the razor cartridge to pivot between the neutral and rotated positions relative to the handle is desirable as this enables the razor cartridge and its associated razor blade(s) to maintain optimal contact with the skin surface being shaved during use.
During normal shaving, many forces act on the razor cartridge. These forces primarily include: frictional forces caused by both the skin contacting elements of the housing and the cutting edge of each razor blade passing over the surface being shaved; forces that result when each razor blade cuts hair and reaction forces from the surface being shaved to elements of the housing and each razor blade edge caused by the user pressing the razor against the skin. It will therefore be apparent that the magnitude and direction of the resultant force acting on the razor cartridge, while being dependent upon the number of razor blades disposed within the cartridge housing, is highly variable. The variability is a result of the individual user's skin type; hair type and hair density, as well as the individual user's shaving habits such as how hard the user presses; how well the user prepares the skin with shaving preparation and what particular shaving preparation, if any, is used.
Pivotal razor cartridges generally have a single, defined pivot axis. The magnitude and direction of the resultant force of the many forces acting on the razor cartridge in use, in combination with the spatial position of the pivot axis relative to the skin surface being shaved can cause a moment to be applied to the razor cartridge. A moment is commonly defined as the measure of the tendency to produce motion about an axis and is calculated as the product of the quantity of force and that force's perpendicular distance from the axis. The moment applied to the razor cartridge in use can be beneficial to shaving or can cause undesirable effects. Undesirable effects include chatter or a tendency for the razor cartridge to rotate away from the skin surface during use. Beneficial effects include the razor cartridge maintaining optimal contact with the skin.
Various structures for pivotally connecting a razor cartridge having two or three razor blades to a handle of a safety razor to provide a defined pivot axis are known in the art. In all cases the pivot axis is parallel to the cutting edge or edges of the razor blades. Typical structures are disclosed in GB Patent number 1460732 to Terry et al, U.S. Pat. No. 4,275,498 to Ciaffone, U.S. Pat. No. 4,970,784 to Althaus, U.S. Pat. No. 5,661,907 to Apprille and several patents to Gilder, particularly U.S. Pat. No. 6,612,040.
It is desired to improve the shaving performance of a safety razor by providing four or more razor blades disposed within the cartridge housing as, for example, is disclosed in U.S. patent application Ser. No. 10/782,173 to Coffin. However, simply accommodating a fourth (or further) razor blade in a widened conventional three blade cartridge housing results in a forces being applied to the cartridge during normal shaving that are different from those that are applied to a two or three blade cartridge.
Based on the foregoing, it is the object of the present invention to provide a pivotal connection structure having an optimized pivot axis location for a razor cartridge having at least four razor blades.